Intralumenally implantable frames can be implanted to treat a variety of medical conditions. Implantable frames can maintain patency of body vessels or provide support for a valve or valve leaflets for regulating fluid flow within a body lumen. For example, flexible leaflet material can be attached to an implantable frame to form a valve prosthesis useful in providing an artificial valve for treating venous valve insufficiency. In addition, a variety of other implantable prostheses, such as stents, stent grafts and the like, comprise a radially expandable support frame placed within the body to improve the function of a body lumen. Support frames may be implanted in vessels, ducts or channels of the human body and can form part of a valve to regulate fluid flow within a body lumen or as scaffolding to maintain the patency of a body vessel, duct or channel to treat various conditions.
Endolumenal prostheses comprising support frames can be placed in a body lumen from a delivery system which includes a catheter. Implantable frames can be intralumenally delivered inside the body by a catheter that supports the stent in a radially compressed form as it is transported to a desired site in a body vessel. Upon reaching the site, the implantable frame can be radially expanded and securably positioned within the lumen of the body vessel, for example by engaging the walls of the body vessel with a portion of the implantable frame. The expansion mechanism may involve expanding the implantable frame radially outward, for example by inflation of a balloon carried by the catheter. Alternatively, when the implantable frame is formed of a self-expanding material, the implantable frame may be delivered in a radially restrained configuration and deployed by removing the restraint at a point of treatment to allow the implantable frame to self-expand by its own internal elastic restoring force at a point of treatment. After expansion of the implantable frame, the catheter delivery system is subsequently withdrawn from the body vessel. Endolumenally implantable support frames preferably possess sufficient hoop strength to resist collapse of the body vessel, while maintaining a desired degree of radial or longitudinal flexibility to prevent damage to the body vessel.
Implantable frames are subjected to various mechanical forces before, during and after deployment within a body lumen. Before deployment, implantable frames can be compressed and maintained in a compacted form, which can include subjecting the implantable frame to a prolonged inward radial restraining force. During deployment, implantable frames can be subjected to an outward radial expanding force, for example from a balloon expansion or self-expansion process. The implantable frames can also be subjected to an inward radial compressive force upon contact with the body vessel wall during deployment expansion. After deployment, implanted frames can be subject to continued inward radial force from the body vessel wall, in addition to a variety of shearing or tortional forces imparted by movement of the body vessel wall or fluid flow within the body vessel. Uneven mechanical load bearing within an implantable frame can result in uneven wear and distortion of the implantable frame shape, or even failure of structural integrity. In typical sinusoidal and near sinusoidal designs, the bends or radial arcs experience areas of high strain and stress, which can lead to areas of frame fatigue or fracture. However, the stress and/or strain experienced along the length of the radial arc may not be uniform, and there are areas of relatively high stress and/or strain. Therefore, it is desirable to provide implantable frames that more evenly distribute mechanical loads.
Dynamic fluctuations in the shape of the lumen of a body vessel, such as a vein, pose challenges to the design of support frames for implantation within the body vessel. For instance, the flow velocity and diameter of veins do not remain essentially constant at a given systemic vascular resistance. Instead, the shape of vein lumens can fluctuate dynamically in response to the respiration, body position, central venous pressure, arterial inflow and calf muscle pump action of a mammalian subject. The veins also provide the principal volume capacitance organ. For example, an increase of almost 100% in the diameter of the common femoral vein has been observed in human patients simply by rotation of the patient by about 40 degrees, corresponding to a four-fold increase in blood flow volume. Moneta et al., “Duplex ultrasound assessment of venous diameters, peak velocities and flow patterns,” J. Vasc. Surg. 8; 286-291 (1988). The shape of a lumen of a vein can undergo dramatic dynamic change as a result of varying blood flow velocities and volumes therethrough, presenting challenges for designing implantable intralumenal prosthetic devices that are compliant to the changing shape of the vein lumen.
Preferably, implantable frames are also configured to minimize undesirable irritation of the lining of a body vessel upon implantation, for example by minimizing the surface area of the frame in contact with the body vessel. However, reducing the surface area of the frame may increase the mechanical stress and strain on particular portions of the frame, particularly bends or arcuate sections. What is needed are endolumenally implantable medical device frames configured to withstand radial compression upon implantation by desirably distributing the associated mechanical strain on the implanted frame, while also minimizing potential irritation of a body vessel that may result from contact between the body vessel and the external surface of the implanted frame.